1. Field of the Invention
The present invention relates to efficiency enhancements for microwave amplification devices, and more particularly, to a novel capacitive stub for adjusting the impedance level across an output gap of a klystron that provides enhanced efficiency and bandwidth for the klystron.
2. Description of Related Art
Linear beam tubes are used in sophisticated communication and radar systems which require amplification of an RF or microwave electromagnetic signal. A conventional klystron is an example of a linear beam microwave amplifier. A klystron comprises a number of cavities divided into essentially three sections: an input section, a buncher section, and an output section. An electron beam is sent through the klystron, and is velocity modulated by an RF electromagnetic input signal that is provided to the input section. In the buncher section, those electrons that have had their velocity increased gradually overtake the slower electrons, resulting in electron bunching. The traveling electron bunches represent an RF current in the electron beam. The RF current induces electromagnetic energy into the output section of the klystron as the bunched beam passes through the output cavity, and the electromagnetic energy is extracted from the klystron at the output section. An output waveguide channels the electromagnetic energy to an output device, such as an antenna.
The development of high powered klystron amplifiers which operate at a peak power level higher in relation to pulse length and frequency than that of conventional klystrons has resulted in beam voltage levels generally higher than that previously achieved. To avoid RF breakdown in the output section due to the high beam voltage, multi-cavity output circuits were developed. The multi-cavity output circuits, known as extended interaction output circuits (EIOC), have the advantage that a higher level of impedance across a greater bandwidth can be achieved, enabling better impedance matching with the electron beam and leading to greater efficiency of operation. An EIOC used to produce high power microwave energy with large instantaneous bandwidth is referred to as an extended interaction klystron (EIK), and can be used to produce power over bandwidths in excess of ten percent. An example of a high performance EIOC is disclosed in U.S. Pat. No. 4,931,695, to Symons.
The function of the output circuit of a klystron or EIK is to convert the kinetic energy of the electron beam into RF power. This is accomplished by generating an impedance level across the output gap (or gaps in the case of an EIK) roughly equivalent to the product of the DC beam impedance and the gap coupling coefficient. The value of the output gap impedance is the product of cavity R/Q (equivalent to the capacitive reactance of the gap, R being the shunt resistance and Q being the quality factor of the cavity) and the Q.sub.total. Since the R/Q is dependent upon gap geometry, it is constrained by a number of factors (most notably the coupling coefficient) and thus is not easily adjusted after assembly. The value of Q.sub.total is the parallel addition of the internal cavity Q (determined by internal resistive losses), beam loaded Q (a complex function of both beam current and velocity modulation), and external Q (dependent upon the degree of coupling to the output waveguide). Varying any of these values will alter the amount of impedance developed across the output gap (or gaps).
Since the resistive losses in the cavity and the modulation on the beam are factors which are not easily modified, one is generally constrained to controlling the cavity Q by concentrating on changing the external Q by adjusting the coupling between the cavity and the output waveguide. This is generally accomplished by use of an inductive waveguide coupling iris which is positioned between the cavity and the output waveguide. The principal drawback to this method is that once the dimensions of the coupling iris are set, it is difficult to further modify the external Q once a completed device is assembled and evacuated. Since the exact level of impedance necessary for maximum efficiency is dependent on many factors, the external Q selected is often less than optimum.
Accordingly, it would be desirable to provide an apparatus for use with a klystron or EIK that enables adjustment of the impedance level across the output gap after assembly and evacuation is complete. Such an apparatus would provide enhanced efficiency and bandwidth for the klystron or EIK. It would be further desirable to provide an apparatus having the above characteristics, while being relatively simple to design and cost effective to fabricate.